﻿<p>The <em>IfcTriangulatedIrregularNetwork</em> is a triangulated face set for representing horizontal surfaces (one unique Z coordinate for all X and Y coordinates within domain) with additional flags for each face indicating breaklines between faces or designation as a hole or void. Triangles shall be defined with vertices in counterclockwise order as viewing from above (following right-hand rule).</p>

<p>For visualization, applications should not display faces where flags are set as negative (either a hole, void, or possible future extension).</p>

<p>The flag <i>Void</i> shall be used to indicate that faces are to be excluded without falling back on any other geometry. Such designation could be used for portions of a site beneath a building or other structure.</p>

<p>The flag <i>Hole</i> shall be used to indicate that faces are to be excluded but may fall back on other geometry. Such designation could be used for portions of a proposed site that are to remain unchanged (conforming to an existing site that may also be defined)</p>

<p>For scenarios where multiple surfaces used as input are to be combined, any triangles marked <i>Void</i> shall be retained as voids, while any triangles marked as <i>Hole</i> shall be overridden if another surface has visible geometry defined within the same horizontal location.</p>